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					Scheduling in Batch Systems




               Three level scheduling
Admission: job mix (long term scheduler)
Memory: degree of multiprogramming (medium term)
CPU Scheduler: algorithm to choose ready process to run
  (short term)
Basic Concepts
 Maximum CPU utilization obtained with
  multiprogramming
 CPU–I/O Burst Cycle – Process execution
  consists of a cycle of CPU execution and
  I/O wait.
 CPU burst distribution
I/O Bursts
Histogram of CPU-burst Times




   Lots of short CPU activities
   Few CPU intensive
               CPU Scheduler
 Selects from among the processes in memory that
  are ready to execute, and allocates the CPU to one of
  them.
 CPU scheduling decisions may take place when a
  process:
  1.Switches from running to waiting state.
  2.Switches from running to ready state.
  3.Switches from waiting to ready.
  4.Terminates.
 Scheduling under 1 and 4 is nonpreemptive.
 All other scheduling is preemptive.
               Dispatcher
 Dispatcher module gives control of the CPU
  to the process selected by the short-term
  scheduler; this involves:
     switching context
     switching to user mode
     jumping to the proper location in the user
      program to restart that program
 Dispatch latency – time it takes for the
  dispatcher to stop one process and start
  another running.
            Scheduling Criteria
 CPU utilization – percentage of time the CPU is
  executing a process     (* more on next slide *)
 Throughput – # of processes that complete their
  execution per time unit
 Turnaround time – amount of time to execute a
  particular process
 Waiting time – amount of time a process has been
  waiting in the ready queue
 Response time – amount of time it takes from when a
  request was submitted until the first response is
  produced, not output (for time-sharing environment)
            CPU Utilization
 Keep the CPU as busy as possible
 Load on system affects level of utilization
     High level of utilization is easier to reach on
      heavily loaded system
 On single-user system, CPU utilization is
  not very important
 On time-shared system, CPU utilization
  may be primary consideration
Scheduling Algorithm Goals
Optimization Criteria
   Max CPU utilization
   Max throughput
   Min turnaround time
   Min waiting time
   Min response time
              Scheduling Algorithms
 Non-preemptive
     Process retains control of CPU until process blocks or is
      terminated
     Good for batch jobs when response time is of little
      concern
     Common: FCFS, SJF
 Preemptive
     Scheduler may preempt a process before it blocks or
      terminates, in order to allocate CPU to another process
     Necessary on interactive systems
     Common: SRT, RR
First-Come, First-Served (FCFS) Scheduling
                   Process Burst Time
                       P1      24
                       P2       3
                       P3       3
     Suppose that the processes arrive in the
      order: P1 , P2 , P3
      The Gantt Chart for the schedule is:

                 P1               P2        P3


        0                    24        27        30
  FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order
                P2 , P3 , P1 .
 The Gantt chart for the schedule is:

          P2       P3        P1


      0        3        6             30



 Waiting time for P1 = 6; P2 = 0; P3 = 3
 Average waiting time: (6 + 0 + 3)/3 = 3
 Much better than previous case.
  Shortest-Job-First (SJF) Scheduling
 Associate with each process the length of its next CPU
  burst. Use these lengths to schedule the process with the
  shortest time. Tie breaker via FCFS.
 Two schemes:
    nonpreemptive – once CPU given to the process it cannot

     be preempted until completes its CPU burst.
    preemptive – if a new process arrives with CPU burst

     length less than remaining time of current executing
     process, preempt. This scheme is know as the SJF-
     preemptive or
     Shortest-Remaining-Time-First (SRT or SRTF).
 SJF is optimal – gives minimum average waiting time for a
  given set of processes.
Example of Non-Preemptive SJF

     ProcessArrival TimeBurst Time
        P1      0.0        7
        P2      2.0        4
        P3      4.0        1
        P4      5.0        4
 SJF (non-preemptive)
        P1          P3       P2        P4


    0    3      7        8        12        16
Example of Preemptive SJF
       Process Arrival Time Burst Time
         P1         0.0         7
          P2        2.0         4
          P3        4.0         1
          P4        5.0         4
 SJF (preemptive)
       P1       P2       P3       P2       P4        P1


   0        2        4        5        7        11        16
                   SJF
 Favors short jobs over long
 Constant arrival of small jobs can lead to
  starvation of long jobs
        Priority Scheduling
 A priority number (integer) is associated
  with each process
     Base on process characteristic (memory usage,
      I/O frequency)
     Base on user
     Base on usage cost (CPU time for higher
      priority costs more)
     User or administrator assigned (static)
     May be dynamic (e.g., changing with amount
      of time running)
 Priority Scheduling (continued)
 The CPU is allocated to the process with the
  highest priority (smallest integer  highest
  priority).
     Preemptive
     nonpreemptive
 SJF is a priority scheduling algorithm where
  priority is the predicted next CPU burst time.
 Problem  Starvation – low priority processes
  may never execute.
 Solution  Aging – as time progresses increase
  the priority of the process.
              Round Robin (RR)
 Each process gets a small unit of CPU time (time quantum),
  usually 10-100 milliseconds. After this time has elapsed,
  the process is preempted and added to the end of the ready
  queue. (Interval timer generates interrupt.)
 If there are n processes in the ready queue and the time
  quantum is q, then each process gets 1/n of the CPU time in
  chunks of at most q time units at once. No process waits
  more than (n-1)q time units.
 Performance
    q large  FIFO

    q small  good response time; however, q must be large

      with respect to context switch, otherwise overhead is too
      high.
Ex. of RR with Time Quantum = 20
           Process Burst Time
               P1        53
               P2        17
               P3        68
               P4        24
    The Gantt chart is:

         P1        P2    P3        P4        P1     P3     P4   P1   P3   P3

     0        20    37        57        77        97 117    121 134 154 162
Time Quantum and Context Switch Time
       Treating All Jobs the Same
 These algorithms basically treat all jobs the
  same
 Each algorithm favors a certain kind of process
 To address this deficiency, multilevel feedback
  queues customize the scheduling of processes
  based on the process’s performance
  characteristics by utilizing 2 or more
  scheduling algorithms
     Flexible
     Complex
                  Multilevel Queue
 Ready queue is partitioned into separate queues:
  foreground (interactive)
  background (batch)
 Each queue has its own scheduling algorithm,
  foreground – RR
  background – FCFS
 Scheduling must be done between the queues.
    Fixed priority scheduling; (i.e., serve all from foreground

     then from background). Possibility of starvation.
    Time slice – each queue gets a certain amount of CPU

     time which it can schedule amongst its processes; i.e.,
     80% to foreground in RR
    20% to background in FCFS
Multilevel Queue Scheduling
     Multilevel Feedback Queue
 A process can move between the various queues; aging
  can be implemented this way.
 Multilevel-feedback-queue scheduler defined by the
  following parameters:
    number of queues

    scheduling algorithms for each queue

    method used to determine when to upgrade a process

    method used to determine when to demote a process

    method used to determine which queue a process will

     enter when that process needs service
Example of Multilevel Feedback Queue
   Three queues:
       Q0 – time quantum 8 milliseconds
       Q1 – time quantum 16 milliseconds
       Q2 – FCFS
   Scheduling
       A new job enters queue Q0 which is served FCFS.
        When it gains CPU, job receives 8 milliseconds. If it
        does not finish in 8 milliseconds, job is moved to
        queue Q1.
       At Q1 job is again served FCFS and receives 16
        additional milliseconds. If it still does not complete,
        it is preempted and moved to queue Q2.
Multilevel Feedback Queues
     Thread Scheduling




Possible scheduling of user-level threads
 50-msec process quantum
 threads run 5 msec/CPU burst
      Thread Scheduling




Possible scheduling of kernel-level threads
 50-msec process quantum
 threads run 5 msec/CPU burst

				
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